1. Field of the Invention
The present invention relates to a beam splitter which transmits light and reflects light in approximately equal quantities of light in the visible region. More particularly, the present invention relates to a low polarization beam splitter which can reduce the difference between polarized light components.
2. Description of the Prior Art
Many beam splitters conventionally used in the art are of the type comprising a dielectric multilayer film or a metallic single layer film. However, these types of beam splitter have various disadvantages. The first mentioned type of beam splitter comprising a dielectric multilayer film has a complicated film structure and is difficult to manufacture although it exhibits no absorption. The second mentioned type of beam splitter comprising a metallic single layer film such as a film of Ag is low in absorption and relatively easy to manufacture. But, the light transmitted through it and the light reflected by it in the visible region are not equal to each other in wavelength characteristics. When a metallic single layer film such as a film of Cr is used, the transmitted light and reflected light of the visible region will have nearly equal wavelength characteristics. However, in this case, the Cr layer has a disadvantage of high absorption.
An important drawback common to the beam splitter comprising a dielectric multilayer film and that comprising a metallic single layer is that the difference between polarized components of transmitted light and reflected light is large. A large difference between polarized light components often causes a problem in practical use of the beam splitter. For example, if a beam splitter having polarization is used in a photometer, then a variance of measured value will be caused by the large difference in polarization between lights from the object. Also, in case of an interferometer of the type in which the light transmitted through a beam splitter is reflected on an object and then reflected on the same beam splitter for observation, there is caused a substantial decrease of light quantity by the polarization of the beam splitter.
An example of low polarization beam splitter is disclosed in U.S. Pat. No. 3,559,090. This known beam splitter is a cube type beam splitter which is manufactured in the following manner:
At first, a dielectric film having a refractive index in the range of from 1.80 to 2.5 is vapour deposited on a prism and then a film of Ag is vapour deposited thereon. Subsequent to it, a second dielectric film having a refractive index in the same range as above is vapour deposited and lastly a prism is cemented thereto.
In view of arrangement of films, the above described beam splitter also involves some problems. Since prisms are used, the optimum refractive index required for the dielectric films to obtain the desired characteristics has to be changed with the change of refractive index of prisms then used. Therefore, there may occur such case where the dielectric film having a suitable refractive index is not available. The above arrangement of films can not be applied to a plate type beam splitter the outer medium of which is air. For a plate type beam splitter it is impossible to obtain the desired characteristics by using dielectric films having a refractive index in the range of from 1.80 to 2.5.